Antigen mediated histamine release form RBL-2H3 cells is associated with substantial hydrolysis of membrane inositol phospholipids, mobilization of intracellular and extracellular Ca2+ the activation of protein kinase C and the phosphorylation of the light and heavy chains of myosin by protein kinase C. The relative importance, however, of each of these events in promoting the secretion of inflammatory substances of these cells has not been clearly defined. Studies with variants of the RBL-2H3 cells revealed several variants that were completely unresponsive to antigen but did release histamine when challenged with a combination of ionophore and phorbol ester. One such variant showed no or very little phosphoinositide hydrolysis in response to stimulants of GTP-regulatory proteins such as sodium fluoride in intact cells and GTPgammaS in permeabilized cells. The studies indicate that these variants will be useful in further defining the relationship between the stimulatory events and secretion in RBL-2H3 cells.